Several different techniques are used in RFID transponders for transmitting data in a downlink mode from a read/write (R/W) unit to the transponder and receiving data in an uplink mode from the transponder at the R/W unit. The present invention relates to a transponder where the uplink signal is, for example, modulated according to an amplitude shift keying (ASK) or frequency shift keying (FSK) principle, and the uplink according to ASK in particular using on-off keying (OON). Using this particular ASK principle, a signal with a specific frequency is transmitted to the transponder and periodically switched on and off. For example, the length of the off periods indicates whether a logic ‘1’ or a logic ‘0’ is transmitted. Therefore, the transponder has to determine the length of an off-period. This can be done with a clock signal generated with an internal crystal oscillator. However, implementing a crystal oscillator in a transponder is rather expensive and therefore not economic for most identification or tagging purposes. Another approach consists of using an LC-oscillator (also LC-tank or resonant circuit). These types of oscillators are cheap. Practically, the inductors and capacitors forming the antenna represent a resonant circuit which is used as an oscillator. The antenna or resonant circuit is then excited by an external RF wave. If the external excitation stops, the oscillation of the antenna (resonant circuit) dies out. This limits the variety of applications. The oscillation can be maintained by use of a resonant circuit, which might use class C amplifiers. Class C amplifiers contain a LC tank or resonant circuit and a plucking device (e.g. a transistor), which is periodically switched on and off for a short time. In this way, the resonant circuit is actively triggered to continue oscillating. This principle is limited in the usable Q range. Q relates to the quality of the resonant circuit which is known as Q or Q factor.
The time during which the oscillation of a resonant circuit can be maintained without plucking (i.e. additional excitation) depends on Q or the Q factor. A low Q indicates that a resonant circuit has high losses and will therefore oscillate for a shorter period of time without excitation than a high Q resonant circuit. However, for broadband transmission, i.e. for high data rates, a low Q oscillator is required. Also, short distance data exchange between the R/W unit and the transponder has an impact on the Q and reduces it. Thus the oscillation of the resonant circuit could hardly be used as a reference to measure the length of excitation pauses. Also, for uplink data transmission, while the R/W unit does not transmit, this oscillation is used as a constant reference clock signal.
It is a general of the present invention to provide a transponder and a method of operating a transponder which allows the transponder to have broader applicability, wider Q range and being less expensive and complex than an internal crystal oscillator.